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Chapter 40: Aminoglycosides

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A 36-year-old woman is being treated with gentamicin in hospital for a gram-negative infection. Her trough blood concentrations are below the known minimum inhibitory concentration (MIC) of the bacteria. An infectious disease consultant is not concerned about this because

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a. the MIC for most bacteria are notoriously too high.

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b. gentamicin plasma concentrations are not an effective way to monitor therapy.

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c. gentamicin has residual bacteriocidal activity which persists after the plasma concentration has fallen below the MIC.

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d. gentamicin is bacteriostatic.

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e. the mechanism of action of gentamicin is inhibition of cell wall synthesis.

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Answer is c. A postantibiotic effect, that is, residual bactericidal activity persisting after the serum concentration has fallen below the MIC, is characteristic of the aminoglycoside antibiotics; the duration of this effect is concentration-dependent.

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A 64-year-old man is suffering from a gas gangrene infection of his right foot due to a mid-calf arterial clot. The bacteria involved is Clostridium perfringens. An aminoglycoside is not a good choice for antibiotic therapy because aminoglycosides are

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a. bacteriostatic.

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b. not active against anaerobic bacteria.

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c. more toxic under anaerobic conditions.

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d. only effective if they are injected directly into anaerobic tissue.

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e. metabolized more rapidly under anaerobic conditions.

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Answer is b. Aminoglycosides must penetrate intracellularly to be effective. Transport of aminoglycosides across the cytoplasmic membrane is an oxygen-dependent active process. Thus, anaerobic bacteria are resistant to the aminoglycosides because they lack the necessary transport system.

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A 26-year-old woman has the diagnosis of bacterial meningitis. The organism cultured is a gram-negative Pseudomonas susceptible to gentamicin. There is concern about treatment of this patient with gentamicin because

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a. aminoglycosides are not generally active against gram-negative bacteria.

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b. aminoglycosides can cause seizures at therapeutic concentrations.

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c. patients with meningitis have a tendency to be more susceptible to the nephrotoxic effects of the aminoglycosides.

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d. aminoglycosides are polar and do not penetrate the central nervous system.

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e. aminoglycosides are rapidly metabolized in the brain and do not reach therapeutic concentrations.

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Answer is d. Because of their polar nature, the aminoglycosides do not penetrate into most cells, the CNS, or the eye. Concentrations of ...

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